Would you please explain to me why the sentence below is considered incorrect according to http://www.TestMagic.com?
"He exercises every day so as to build his stamina."
The correction is as folllows:
"He exercises every day in an effort to build his stamina."
Doesn't "so as (not) to do sth" equal "in order (not) to do sth", i.e. "because"?
Thank you very much in advance.

You are right to say that "so as (not) to do something" means "in order (not) to do something", or "because".

There is a slight difference in meaning: “in an effort to” indicates that he is trying; whereas "so as to” can be interpreted as an intention. If you choose to use “because”, the meaning is the same as “in order to” but the sentence structure is different. The correct formulation is: "He exercises every day because he wants / needs to build his stamina."

Both “so as to” and “in order to” are correct. However, the test on TestMagic.com is a computer test. Such tests only accept the answers that are entered at the time of programming. It is possible that the test writer wanted to test the use of “in an effort to” and didn’t programme “so as to” as a correct answer.

You are right to say that "so as (not) to do something" means "in order (not) to do something", or "because".

There is a slight difference in meaning: “in an effort to” indicates that he is trying; whereas "so as to” can be interpreted as an intention. If you choose to use “because”, the meaning is the same as “in order to” but the sentence structure is different. The correct formulation is: "He exercises every day because he wants / needs to build his stamina."

Both “so as to” and “in order to” are correct. However, the test on TestMagic.com is a computer test. Such tests only accept the answers that are entered at the time of programming. It is possible that the test writer wanted to test the use of “in an effort to” and didn’t programme “so as to” as a correct answer.